SOS mutator activity: unequal mutagenesis on leading and lagging strands

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• Mutagenesis in Escherichia coli is significantly influenced by the inducible SOS response.
• Current models propose RecA and UmuD'(2)C protein interactions with stalled replication complexes mediate error-prone DNA synthesis.
• The exact mechanisms of SOS-mediated DNA lesion bypass and mismatch extension remain unclear.

Purpose of the Study:

• To investigate differential effects on replication fidelity between leading and lagging strands during constitutive SOS activation (SOS mutator) in E. coli.
• To elucidate the strand-specific mechanisms underlying SOS mutator activity.

Main Methods:

• Utilized recA730 E. coli strains exhibiting constitutive SOS response and a spontaneous mutator effect.
• Constructed strain pairs with differing lac operon orientations relative to the replication origin to assess leading vs. lagging strand replication.
• Employed mismatch-repair defective (mutL) strains to efficiently detect replication errors in the lacZ gene.

Main Results:

• Observed significant differences in lac mutant frequencies between the two orientations, indicating strand-specific mutagenesis.
• Found that the direction of the mutation frequency bias was reversed compared to normal E. coli cells.
• Identified the lagging strand as being particularly vulnerable to the SOS mutator effect.

Conclusions:

• SOS mutator activity exhibits significantly different efficiencies on the leading and lagging strands of DNA replication.
• The lagging strand is disproportionately affected by the SOS mutator effect, leading to increased mutagenesis.
• These findings provide critical insights into the strand-biased mechanisms of DNA repair and mutagenesis in E. coli.